# is λ=h/mv the same equation as λ=hc/E ?

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#2

Someone correct me if I'm wrong, but from what I remember:

λ=h/p

from mechanics: p=mv

so: λ=h/mv

Photons move at the speed of light, so v=c

λ=h/mc

At the same time, from Einstein: E=mc^2

m=E/c^2

thus: λ=h/[(E/c^2)c]

λ=h/(E/c)

λ=hc/E

So basically both equations are related, but not quite the same. You need to apply them depending on what variables are known and which ones you are trying to find.

I hope that helps.

λ=h/p

from mechanics: p=mv

so: λ=h/mv

Photons move at the speed of light, so v=c

λ=h/mc

At the same time, from Einstein: E=mc^2

m=E/c^2

thus: λ=h/[(E/c^2)c]

λ=h/(E/c)

λ=hc/E

So basically both equations are related, but not quite the same. You need to apply them depending on what variables are known and which ones you are trying to find.

I hope that helps.

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#3

(Original post by

Someone correct me if I'm wrong, but from what I remember:

λ=h/p

from mechanics: p=mv

so: λ=h/mv

Photons move at the speed of light, so v=c

λ=h/mc

At the same time, from Einstein: E=mc^2

m=E/c^2

thus: λ=h/[(E/c^2)c]

λ=h/(E/c)

λ=hc/E

So basically both equations are related, but not quite the same. You need to apply them depending on what variables are known and which ones you are trying to find.

I hope that helps.

**ibs2012**)Someone correct me if I'm wrong, but from what I remember:

λ=h/p

from mechanics: p=mv

so: λ=h/mv

Photons move at the speed of light, so v=c

λ=h/mc

At the same time, from Einstein: E=mc^2

m=E/c^2

thus: λ=h/[(E/c^2)c]

λ=h/(E/c)

λ=hc/E

So basically both equations are related, but not quite the same. You need to apply them depending on what variables are known and which ones you are trying to find.

I hope that helps.

But, what you can do is apply and , where is the frequency and is the wavelength.

EDIT:

(Original post by

i am now confused so they are the same or not if not why and when do we use each?!

**>>MMM<<**)i am now confused so they are the same or not if not why and when do we use each?!

What I did is to apply the equation for energy level and the equation relating the speed of wave, wavelength and frequency , because it is a photon, you have , hence

If you have something with mass, you can, of course, apply or for relativistic condition. However, you are considering photon, for momentum, you have .

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(Original post by

Photon is massless, so you can't really apply .

But, what you can do is apply and , where is the frequency and is the wavelength.

**agostino981**)Photon is massless, so you can't really apply .

But, what you can do is apply and , where is the frequency and is the wavelength.

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#5

**agostino981**)

Photon is massless, so you can't really apply .

But, what you can do is apply and , where is the frequency and is the wavelength.

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#6

**>>MMM<<**)

i am now confused so they are the same or not if not why and when do we use each?!

In that case the only use I can find for λ=h/mv is when you are trying to find the de Broglie wavelength of larger objects, ie a car or person moving.

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(Original post by

I'm assuming that your variables are m=mass,v=velocity?

In that case the only use I can find for λ=h/mv is when you are trying to find the de Broglie wavelength of larger objects, ie a car or person moving.

**ibs2012**)I'm assuming that your variables are m=mass,v=velocity?

In that case the only use I can find for λ=h/mv is when you are trying to find the de Broglie wavelength of larger objects, ie a car or person moving.

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#8

(Original post by

how is that possible? i have read in my book that λ=h/mv can be used in any situation?

**>>MMM<<**)how is that possible? i have read in my book that λ=h/mv can be used in any situation?

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#9

**ibs2012**)

Someone correct me if I'm wrong, but from what I remember:

λ=h/p

from mechanics: p=mv

so: λ=h/mv

Photons move at the speed of light, so v=c

λ=h/mc

At the same time, from Einstein: E=mc^2

m=E/c^2

thus: λ=h/[(E/c^2)c]

λ=h/(E/c)

λ=hc/E

So basically both equations are related, but not quite the same. You need to apply them depending on what variables are known and which ones you are trying to find.

I hope that helps.

λ=hc/E is the wavelength of a photon with a certain energy. You usually see the equation in this form: E=hc/λ. and they will usually ask to find the wavelength energy given a certain wavelength.

I dont think they are they same (they might be, but I dont' know). But generally in my course (OCR), λ=h/mv is for particles and E=hc/λ is for photons of light.

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(Original post by

λ=h/mv is known as the Broglie wavelength and you use it when you are asked to find the associated wavelength of a particle such as an electron with a certain speed.

λ=hc/E is the wavelength of a photon with a certain energy. You usually see the equation in this form: E=hc/λ. and they will usually ask to find the wavelength energy given a certain wavelength.

I dont think they are they same (they might be, but I dont' know). But generally in my course (OCR), λ=h/mv is for particles and E=hc/λ is for photons of light.

**bbcs2k9**)λ=h/mv is known as the Broglie wavelength and you use it when you are asked to find the associated wavelength of a particle such as an electron with a certain speed.

λ=hc/E is the wavelength of a photon with a certain energy. You usually see the equation in this form: E=hc/λ. and they will usually ask to find the wavelength energy given a certain wavelength.

I dont think they are they same (they might be, but I dont' know). But generally in my course (OCR), λ=h/mv is for particles and E=hc/λ is for photons of light.

**that**mean?

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#11

(Original post by

that was the answer i was looking forward to but the problem is that my book says that a beam of moving electrons has associated de broglie wavelength (where at high speed you have to take into account relativistic effects) is approx λ≃hc/E can you please tell me what does

**>>MMM<<**)that was the answer i was looking forward to but the problem is that my book says that a beam of moving electrons has associated de broglie wavelength (where at high speed you have to take into account relativistic effects) is approx λ≃hc/E can you please tell me what does

**that**mean?
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(Original post by

If you want to calculate the relativistic de broglie wavelength of a particle of rest mass then the equation becomes:

**theterminator**)If you want to calculate the relativistic de broglie wavelength of a particle of rest mass then the equation becomes:

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#13

(Original post by

thanks but how can we use λ=hc/E to find the de broglie of electrons if it is primarily used for photons (where E is the energy of the electron)?!

**>>MMM<<**)thanks but how can we use λ=hc/E to find the de broglie of electrons if it is primarily used for photons (where E is the energy of the electron)?!

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#14

**>>MMM<<**)

thanks but how can we use λ=hc/E to find the de broglie of electrons if it is primarily used for photons (where E is the energy of the electron)?!

*particle*, you need to use λ=h/mv.

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#15

**>>MMM<<**)

thanks but how can we use λ=hc/E to find the de broglie of electrons if it is primarily used for photons (where E is the energy of the electron)?!

If you want to find the de-Broglie wavelength of electrons, you need .

If you seek a non-relativistic result, you apply , if you seek a relativistic result, you apply , where

A pretty obvious reason why you can't use is because electrons cannot reach light speed.

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(Original post by

Nope, is for photons only.

If you want to find the de-Broglie wavelength of electrons, you need .

If you seek a non-relativistic result, you apply , if you seek a relativistic result, you apply , where

A pretty obvious reason why you can't use is because electrons cannot reach light speed.

**agostino981**)Nope, is for photons only.

If you want to find the de-Broglie wavelength of electrons, you need .

If you seek a non-relativistic result, you apply , if you seek a relativistic result, you apply , where

A pretty obvious reason why you can't use is because electrons cannot reach light speed.

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(Original post by

Look again at bbcs2k9's post above. E=hf (or E=hc/λ, they are the same equation) is for photons. If you wnat to find the deBroglie wavelength of a

**Pangol**)Look again at bbcs2k9's post above. E=hf (or E=hc/λ, they are the same equation) is for photons. If you wnat to find the deBroglie wavelength of a

*particle*, you need to use λ=h/mv.__only__for particles and λ=hc/E can

__only__for photons and the book has some kind of mistake in it?

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(Original post by

I think you may be getting confused with the derivation of the de Broglie equation from above. The above derivation is the one taught but isn't actually the correct way of deriving the de Broglie wavelength. The correct derivation requires more fundamental special relativity but it makes the clear distinction between photons and electrons. Something you should know about the de Broglie wavelength is that it is a relativistic effect, and E=mc^2 is an equation for something that has zero velocity in the frame of reference being used.

**theterminator**)I think you may be getting confused with the derivation of the de Broglie equation from above. The above derivation is the one taught but isn't actually the correct way of deriving the de Broglie wavelength. The correct derivation requires more fundamental special relativity but it makes the clear distinction between photons and electrons. Something you should know about the de Broglie wavelength is that it is a relativistic effect, and E=mc^2 is an equation for something that has zero velocity in the frame of reference being used.

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#19

(Original post by

soo your saying λ=h/mv is can

**>>MMM<<**)soo your saying λ=h/mv is can

__only__for particles and λ=hc/E can__only__for photons and the book has some kind of mistake in it?
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#20

(Original post by

so λ=hc/E can't be used for particles??

**>>MMM<<**)so λ=hc/E can't be used for particles??

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